The present disclosure relates generally to ultrasonic, non-destructive testing methods and, more particularly, to an ultrasonic inspection method and system for determining incipient mechanical failure.
Many mechanical failure modes include a long-duration first step in which microstructural damage and/or change accumulates in a region, followed thereafter by occurrence of observable cracks and failure. Of the overall service lifetime of a part, only a small amount of life remains once cracks are observable.
Cracks that are above certain threshold sizes, and within certain specified regions, may be detected by existing ultrasound or eddy current techniques. For example, in conventional ultrasound harmonic imaging, ultrasound signals or pulses are transmitted at fundamental frequencies, and echo signals are received by a transducer. Discontinuities, such as cracks, can be detected when their echoes are greater than that of the background noise.
Unfortunately, by the time a crack can be detected through such methodologies, the part has essentially failed. For example, fatigue cracks in titanium objects become detectable when only about 10% of life is remaining. The presence of an identified crack signifies the part has exhausted its life. Additionally, the presence of cracks in a particular part may prevent that part from being repaired and returned to service. Thus, it would be desirable to be able to detect incipient damage while the part is still repairable.
There are at least two noteworthy applications in which incipient mechanical failure analysis can be applied. The first relates to detection of incipient dwell-time fatigue in titanium alloy aircraft engine compressor forgings, and the second relates to detection of creep damage in structural applications such as aircraft engine and land gas turbine airfoils and disks. Dwell-time fatigue arises from the anisotropy of modulus and limited slip systems in titanium. Thus, if cyclic stresses (near the yield stress) are applied with hold times to a titanium body, then grains elastically deform to different degrees due to their individual crystallographic orientation with respect to the applied stress. In addition, some grains may begin plastic yielding while others do not. This process applied cyclically can lead to buildup of high stresses at grain (or colony) boundaries. An unfavorably oriented grain or colony of grains can crack by cleavage, wherein such a cleavage crack will lead to premature failure of the part. However, dwell-time fatigue cannot be detected by current techniques until there is a crack present.